The present invention relates to a gas-generant-molded-article which is combusted to form gas components in order to expand an air bag system, and a process for producing the same. More specifically, the present invention relates to a novel gas generant composition that produces operating gases in air bag systems that are carried in automobiles and aircraft and used for protecting human bodies.
Air bag systems are known in which a bag is quickly expanded by gas to prevent occupants from violently colliding against damaging spots and/or hard parts inside vehicles (such as handles and windshields) by inertia when vehicles such as automobiles collide at a high speed. Requirements for gas generants used for air bag systems are very severe, such that bag expansion time is very short, usually 40 to 50 milliseconds, and, further, the gaseous atmosphere in the bag should be harmless to a human body (e.g., close to the air composition in a car).
At present, gas generants usually used for air bag systems include inorganic azide compounds, particularly sodium azide. Sodium azide does not satisfy the requirements described above in terms of safety to occupants since an alkali component which is produced as a by-product in the generation of gas shows toxicity, though sodium azide is excellent in terms of combustibility. Further, since sodium azide itself also shows toxicity, influences that it exerts on the environment when it is thrown away are also of concern.
In order to overcome these defects, some, so-called non-azide gas generants have been developed and substituted for sodium azide gas generants. For example, a composition comprising, as its principal components, tetrazole, triazole or metal salts thereof, and oxygen-containing oxidizing agents such as alkaline metal nitrates is disclosed in JP-A-3-208878. Further, gas generants comprising, as their principal components, metal salts of bitetrazole compounds containing no hydrogen are disclosed in JP-B-64-6156 and JP-B-64-6157.
Furthermore, a gas generant containing a transition metal complex of tetrazole or triazole is shown in JP-B-6-57629. Also, a gas generant containing triaminoguanidine nitrate is shown in JP- A-5-254977; a gas generant containing carbohydrazide is shown in JP-A-6-239683; and a gas generant containing nitrogen-containing non-metal compounds including cellulose acetate and nitroguanidine is shown in JP-A-7-61855. Further, the use of nitroguanidine as an energy material which coexists with 15 to 30% of a cellulose binder is disclosed in U.S. Pat. No. 5,125,684. Furthermore, a gas generant composition comprising a combination of tetrazole and triazole derivatives with an oxidizing agent and a slag-forming agent are disclosed in JP-A-4-265292.
However, nitrogen-containing organic compounds have a defect in that they usually generate a large amount of heat during combustion, as compared with azide compounds, when an oxidizing agent sufficient for generating oxygen in an amount corresponding to the chemical equivalent thereof is used (that is, in an amount necessary for combusting carbon, hydrogen and other elements contained in the molecule of the compound). Although it is essential in an air bag system that the system itself have such a size that it is not obstructive in ordinary driving, in addition to the performance of a gas generant, a large calorific value of a gas generant in combustion requires the presence of an optional part for removing heat when designing a gas generator and therefore makes it impossible to miniaturize the gas generator itself. Although a calorific value can also be reduced by selecting the kind of oxidizing agent, linear burning velocity is also reduced accordingly, which results in reduction in gas generating performance.
As described above, a gas generant composition comprising a nitrogen-containing organic compound has had the defect that it usually generates a large amount of heat in combustion, as compared with gas generant compositions using inorganic azide compounds, when an oxidizing agent sufficient for generating oxygen in an amount corresponding to the chemical equivalent thereof is used. As a result of the combustion temperature being high, the linear burning velocity is small.
A problem caused by high combustion temperatures is that bags are damaged by having released out of an inflater (i) a chemical reaction product of alkaline mists generated from the oxidizing agent components contained in the compositions together with (ii) high temperature hot grains that are newly generated in a cooling part by an erosion of a coolant, which is made of stainless steel in many cases. However, if one could also form a slag in the combustion chamber before the mists and hot grains arrive at the cooling part, this could prevent the alkaline mists generated from oxidizing agent components and high temperature hot grains that are newly generated in a combustion chamber from exiting the inflater. In this way, an inflater system using a small amount of a coolant could be realized without fatally damaging the bag, since the generated gases while having high temperatures also have a small heat capacity. Such an achievement also would make it possible to realize an inflater having a smaller size.
Non-azide gas generant compositions using various nitrogen-containing organic compounds including tetrazole derivatives have previously been investigated. Although the linear burning velocities of the compositions vary depending upon the kind of the oxidizing agent combined therewith, almost all such compositions have a linear burning velocity of 30 mm/second or slower.
The linear burning velocity influences the physical form of a gas generant composition for satisfying required performances. In one form of a gas generant composition, the combustion time of the gas generant composition is determined depending upon the smallest thickness of the thicknesses in a thick part thereof and the linear burning velocity of the gas generant composition. A bag expanding time required of inflater systems is about 40 to 60 milliseconds.
In order to completely combust within this time a gas generant composition having a pellet form and one having a disc form are used in many cases. However, a time of 100 milliseconds is required, for example, when the linear burning velocity is 20 mm/second at a thickness of 2 mm, and therefore the required inflater performance for a vehicle air bag cannot be satisfied.
Accordingly, in a gas generant composition having a linear burning velocity of about 20 mm/second, the performances cannot be satisfied when the thickness thereof is not about 1 mm. Thus, in the case where the linear burning velocity is about 10 mm/second or less, it is an essential condition that the thickness of the thick part is even smaller.
Although a means of combining an oxidizing agent such as sodium nitrate and potassium perchlorate therewith in order to increase the linear burning velocity has been known, sodium oxide from sodium nitrate or potassium chloride from potassium perchlorate is released to outside the inflater in a form of a liquid or solid fine powder, and in the case where a slag-forming agent is not present, it is extremely difficult to reduce the amount thereof to be released to an allowable level by conventional filters.
In order to achieve a thickness of a thick part in a pellet form or a disc form used in many cases when the linear burning velocity is about 10 mm/second or less, the thickness of about 0.5 mm or less is essential. However, it is practically almost impossible to produce a gas generant composition having such a thickness when the same is in a pellet form or a disc form, such that it withstands the vibration of automobiles over a long period of time and is industrially stable.
Extensive investigations repeated by the present inventors in order to solve the problems described above have resulted in the discovery of a novel gas generant composition having a small linear burning velocity, which can be combusted within a specified time by molding it into a specified configuration, and wherein the performance thereof is sufficiently applicable as a gas generant for air bags. Based on this discovery, the present invention has been completed.
That is, in one embodiment the present invention provides a gas-generant-molded-article for air bags which is prepared by molding a gas generant composition into a cylindrical form having an opening hole therein or therethrough, wherein the relationship between the linear burning velocity r (mm/second) of said gas generant composition under a pressure of 70 kgf/cm2 and a thickness W (mm) falls within a range represented by 0.005xe2x89xa6W/(2xc2x7r)xe2x89xa60.3, preferably 0.005xe2x89xa6W/(2xc2x7r)xe2x89xa60.1, and a gas-generant-molded-article for air bags which is prepared by molding a gas generant composition having a linear burning velocity within the range of preferably from 1 to 12.5 mm/second, still more preferably from 5 to 12.5 mm/second under a pressure of 70 kgf/cm2. In the case of describing a linear burning velocity in the present description, this means the velocity under a pressure of 70 kgf/cm2.
In another embodiment, the invention provides for a novel gas generant composition for air bags, which composition comprises a nitrogen containing organic compound, an oxidizing agent, optionally a slag forming agent, and a binder. The provided composition can advantageously be used in preparing a gas-generant-molded-article for air bags according to the present invention.